mixer/wsplice: cmd/bench/main.go; 29 LoC

Found a possible issue in [mixer/wsplice](https://www.github.com/mixer/wsplice) at [cmd/bench/main.go](https://github.com/mixer/wsplice/blob/f126361e2401fd188893240bb62d16d863abf5f6/cmd/bench/main.go#L69-L97)

Below is the message reported by the analyzer for this snippet of code. Beware that the analyzer only reports the first
issue it finds, so please do not limit your consideration to the contents of the below message.

> range-loop variable payload used in defer or goroutine at line 78

[Click here to see the code in its original context.](https://github.com/mixer/wsplice/blob/f126361e2401fd188893240bb62d16d863abf5f6/cmd/bench/main.go#L69-L97)

Click here to show the 29 line(s) of Go which triggered the analyzer.

```go
for _, payload := range payloads {
var (
latency, samples int64
stopped int64
wg sync.WaitGroup
)
for _, conn := range conns {
wg.Add(1)
go func(conn net.Conn) {
buffer := make([]byte, 0, len(payload))
for atomic.LoadInt64(&stopped) == 0 {
start := time.Now()
conn.Write(payload)
io.ReadFull(conn, buffer)
atomic.AddInt64(&latency, int64(time.Now().Sub(start)))
atomic.AddInt64(&samples, 1)
time.Sleep(time.Millisecond * 5)
}
wg.Done()
}(conn)
}

time.Sleep(benchDuration)
atomic.StoreInt64(&stopped, 1)
wg.Wait()

final := float64(latency/int64(time.Microsecond)) / float64(samples) / 2
fmt.Printf("clients=%d, payload=%db, latency=%.1fus\n", clientsCount, len(payload), final)
}

```

Leave a reaction on this issue to contribute to the project by classifying this instance as a **Bug** :-1:, **Mitigated** :+1:, or **Desirable Behavior** :rocket:
See the descriptions of the classifications [here](https://github.com/github-vet/rangeclosure-findings#how-can-i-help) for more information.

commit ID: f126361e2401fd188893240bb62d16d863abf5f6